Recently, in some audio sources such as DVD, multi-channel audio signals of 5.1 channels or the like are recorded. Digital surround-sound systems for reproducing such audio sources have been dominating even in general homes. FIG. 10 is a plan view showing an example of a speaker layout in a digital surround-sound system, in which Zone represents a listening room where surround-sound is reproduced; U, a listening position; SP-L and SP-R, main speakers for reproducing main signals L (left) and R (right); SP-C, a center speaker for reproducing a center signal C (center); SP-SL and SP-SR, rear speakers for reproducing rear signals SL (rear left) and SR (rear right); SP-SW, a subwoofer for reproducing a subwoofer signal LFE (lower frequency); and MON, a video apparatus such as a television set or the like.
According to the digital surround-sound system in FIG. 10, an effective sound field can be created. In the digital surround-sound system, however, a plurality of speakers are disposed to disperse in the listening room Zone so that the rear speakers SP-SL and SP-SR for surround sound are disposed at the rear of the listening position U. Thus, there are drawbacks that the speaker lines of the rear speakers SP-SL and SP-SR become long, and that the layout of the rear speakers SP-SL and SP-SR is bound by the shape of the listening room Zone, furniture, etc.
As a means for relaxing such drawbacks, there has been proposed a surround-sound system in which highly directional speakers are disposed in front of the listening position in place of the rear speakers, and acoustic reflectors are disposed at the rear of the listening position so that surround-channel sounds radiated from the directional speakers are reflected by the acoustic reflectors so as to obtain the same effect as that by the rear speakers disposed at the rear of the listening position (for example, see Patent Document JP-A-06-178379). A method in which wall surfaces at the rear of the listening position are used as acoustic reflectors can be also considered.
A delay array system has been known as a system for controlling the directivities with which sounds are radiated to acoustic reflectors or wall surfaces. The principles of the array speaker will be described below with reference to FIG. 11. A large number of miniaturized speakers 101-1 to 101-n are disposed one-dimensionally. Assume that an arc whose distance from a position (focus) P of the wall surfaces or the acoustic reflectors is L is Z. Extend straight lines connecting the focus P with the speakers 101-1 to 101-n respectively. Consider that virtual speakers 102-1 to 102-n as shown by the broken lines in FIG. 11 are disposed on the intersection points where these extended straight lines intersect the arc Z. Since all the distances between these virtual speakers 102-1 to 102-n and the focus P are L, sounds simultaneously radiated from the speakers 102-1 to 102-n arrive at the focus P simultaneously.
In order that a sound radiated from each real speaker 101-i (i=1, 2, . . . n) is made to arrive at the focus P simultaneously, it will go well if a delay (time difference) corresponding to a distance between the speaker 101-i and a virtual speaker 102-i corresponding thereto is added to the sound output from the speaker 101-i. That is, control is made so that a listener located in the focus P can feel as if the virtual speakers 102-1 to 102-n were disposed on the arc Z. In this manner, the phases of the outputs of the speakers 101-1 to 101-n can be tuned up in the focus P so as to create a mountain of sound pressure. As a result, a sound pressure distribution with directivity felt as if acoustic beams are emitted toward the focus P can be obtained.
When the speakers are disposed not one-dimensionally but two-dimensionally, acoustic beams with three-dimensional directivity can be output. The array speaker has an advantage in that sounds corresponding to a plurality of audio signals respectively can be radiated with different directivities simultaneously, that is, acoustic beams of a plurality of channels can be output simultaneously. Patent Document JP-T-2003-510924 has proposed a multi-channel surround-sound system using an array speaker. When the array speaker is used, a 5.1-channel surround-sound system can be produced by the array speaker alone as shown in FIG. 12. In FIG. 12, SP-L′ and SP-R′ designate virtual main speakers formed in left and right wall surfaces, and SP-SL′ and SP-SR′ designate virtual rear speakers formed in a rear wall surface.
While having the advantage as described above, surround-sound systems using an array speaker also have some problems in practical use. The first problem is the point that the sound image fixed-positions of the main channels (main signals L and R) are wrong. In a surround-sound system using an array speaker, main signals L and R are radiated from the array speaker toward the left and right walls as shown in FIG. 12. Due to sounds reflected by the left and right walls, the listener feels as if sound sources, that is, virtual main speakers SP-L′ and SP-R′ were located near the walls. However, the layout where the virtual main speakers SP-L′ and SP-R′ are disposed in the left and right wall surfaces as shown in FIG. 12 differs from the general layout of speakers shown in FIG. 10. Therefore, the reproducing environment differs from the environment intended by a creator of contents. Particularly in the case of old contents including no center signal C, a sound image to be fixed on a screen is expected to be obscure. Such a problem becomes more conspicuous in a room which is left-right asymmetric or a room which is long from side to side.
The second problem is the point that the sense of the sound image fixed-positions of the surround channels (rear signals SL and SR) are wrong. The rear signals SL and SR avoiding the listening position U and reflected by the left and right walls or the ceiling or by both the left and right walls and the ceiling are reflected by the rear wall and arrive at the listening position U. Thus, the listener feels the sound image fixed-positions at the rear of the listener. In fact, however, each acoustic beam merely creates an intensive directivity distribution. Each acoustic signal spreads in any direction other than the beam direction. The energy in any direction other than the beam direction is merely weaker than the energy in the beam direction. Accordingly, when a direct sound from the array speaker is not much weaker than its beam traveling via the wall, the sound image fixed-position is felt to be closer to the array speaker. Any surround channel has a larger distance from the listener than any main channel. When the distance to the listener is larger, the energy of an audio signal is attenuated disadvantageously to the ratio to the direct sound. In addition, when the distance is larger, it takes more time to arrive at the listening position U. Thus, the sound image is apt to be fixed on the direct sound side due to the Hass effect.
Particularly, there is a problem in difficulty to control a low frequency. The main lobe width of directivity which is the thickness of the acoustic beam depends on the ratio between the wavelength of a signal and the width of the array speaker. Therefore, a high frequency signal forms a narrow beam, and a low frequency signal forms a wide beam. That is, the directivity varies in accordance with the frequency. In order to form an audio signal of one frequency band into a beam, the array width has to be several times as long as the wavelength of the signal. For example, when the frequency is 500 Hz, the wavelength is about 60 cm. The required array width is about 2 m, which is not the practical size for general home use. In such a manner, since intensive directivity cannot be given to a low-frequency signal, the energy of a direct sound overcomes the energy of a reflected beam. Accordingly, a high-frequency signal is fixed on the rear wall side while a low-frequency signal is listened to directly from the array speaker. Thus, the sound image may be separated, or the sense of fixation thereof may be wrong.